25W cl. A amp with Lateral MOSFETs

[juma]

This time I wanted an amp that will combine qualities of my F5 (made with k2013/j313) with a certain seductive gentlenes that comes with good tube amps (member p18 recently made a nice amp with 6C33C that inspired me). Comparatively, I wanted something like F3 but with more control, more gain, more power and insensitiveness to characteristics of source and speakers.
I worked for a few months on it and tried a number of ideas (best candidates were a circlotron with LU1014 and a single rail version of F5 with BJTs at input and Laterals at output) but the buck stopped here:

I chose single rail PS becouse it enables using the benign reactive component at the output that makes "sound tailoring" possible.
Transformer is 500VA (2 x 40V AC, one secandary per channel). Cap. multiplier provides smooth and fast source of power (1.5mV ripple at 1.3A). 10mf ELKOs are Siemens Sikorel, bypass are MKC in PS and MKP in direct signal path.
Q1 provides high Zin and Low Z feedback point. Q2 gives all the voltage gain (OLG is about 50dB), it runs on about 7.5mA and that's more than enough to drive a pair of Laterals in push-pull Source follower topology (there are jumpers over Source resistors which are taken off when setting the bias with P2 ; P1 sets the half of the PS voltage at the output).
Here is the 90kHz square wave at the output loaded with Epos 5M (two way, bass reflex, 4 Ohm speaker). CRO was set at 5V/div vertical and 2uSec/div horizontal:


Construction looks like this:


and the rest of the pictures are here:
Projekat: SVAJA - diayudio.rs

[magnoman]

Juma

Wow that's some response. Interesting arrangement of your Q2 stage (to me anyway). Thanks for sharing, got me thinking of a few different circuits which just might benefit from a similar configuration.

Thanks
-Tony

[AndrewT]

Hi,
what does R14 do?

[Mooly]

Nice to see something different... you might even say a bit of lateral thinking even

At the risk of sounding picky, 7.5 ma just sounds rather a lot to feed through a standard preset wiper for long term (many years) reliability. Fixed resistors maybe ?

And I can tell you now how this amp will sound... wonderful!!!

[gaborbela]

Congrats Juma
Nice work

[juma]

Originally Posted by AndrewT

Hi,
what does R14 do?

R14 bleeds C2

[juma]

Originally Posted by Mooly

...7.5 ma just sounds rather a lot to feed through a standard preset wiper for long term (many years) reliability....

7.5ma x 5V is about one third of the pot's power rating so it's OK.

[apexaudio]

Great work, I like this 'le retro' circuit, can I suggest to put outputs on pcb in future. I expect many diyers will build this amp. Nice way to use laterals.
Regards

[juma]

Originally Posted by magnoman

... Interesting arrangement of your Q2 stage...

Actually it's a very common way to make a Vas stage. It's just that I carefully trimmed it (bootstrap configuration and emitter degeneration) to achieve the ammount of gain (about 50dB OLG) that gave the sonic result that I like the best.

[juma]

Originally Posted by apexaudio

Great work, I like this 'le retro' circuit, can I suggest to put outputs on pcb in future. I expect many diyers will build this amp. Nice way to use laterals.
Regards

Thanks Mile (and all the other guys)

There is a PCB under the aluminum profile where the MOSFETs are mounted. This way it was easy to test different front stages.